Compression mold for forming a composite material fixed angle rotor

ABSTRACT

A method and apparatus for the compression molding of composite fiber fixed angle rotors is disclosed. A female mold member defines a closed cylinder cavity for molding the bottom surface of the rotor, this cavity usually defining a frustum shaped central cavity complimentary to and concentric with the spin axis of the ultimately formed rotor. A male mold member having a complimentary cylindrical profile contains a frustum shaped inner cavity with the apex of the frustum disposed to the inner portion of the cylinder and the base end of the frustum exposed to the cylindrical opening of the female mold. This frustum shaped inner cavity defines the exterior frustum shape of the ultimately produced rotor and defines between the exterior frustum profile and the frustum shaped inner cavity a rotor body wall having sufficient thickness to receive the sample tube apertures. At the apex end of the frustum shaped cavity in the male mold member, there is located a lock system for maintaining sample tube aperture cores. These sample tube aperture cores are locked within the frustum cavity in the precise alignment of the ultimately formed sample tubes of the rotor. Loading of the mold with resin pre-impregnated fiber typically occurs in the frustum shaped cavity of the male mold member and at the bottom of the female mold member. A compression molded rotor product is disclosed having discontinuous fibers optimally disposed to resist the forces of centrifugation.

This is a Division of application Ser. No. 08/431,544 filed May 1, 1995and now U.S. Pat. No. 5,643,168 the disclosure of which is incorporatedby reference.

This invention relates to composite material centrifuge rotors of theso-called "fixed angle" variety. More particularly, a method andapparatus for the compression molding of a fixed angle rotor isdisclosed.

BACKGROUND OF THE INVENTION

Fixed angle centrifuge rotors are known. In such rotors, sample tubeapertures of the rotor are disposed at a "fixed angle" in the normalrange of 20° to 34°. Material to be centrifugated is placed in sampletubes within the sample tube apertures in the rotor body and spun athigh speed. Classification of the material within the sample tubesoccurs. At the end of such centrifugation, the classified sample iswithdrawn and further processed.

It is known to make fixed angle rotors from composite materials.Further, it has been suggested to make such fixed angle rotors withchopped or discontinuous fibers. Unfortunately, fiber alignment of suchchopped or discontinuous fibers has heretofore not been possible.

It is known that composite materials have anisotropic strength ofmaterial properties. Specifically, such materials have great resistanceto tension, but are generally poor in resistance to all other modes ofloading. In order to take maximum advantage of the tensile strength ofsuch fibers, fiber alignment to a disposition where stresses ofcentrifugation can be resisted is required. This usually--but notalways--requires that the fibers be aligned either normal to the spinaxis or radially about the spin axis.

Compression molding of composite fiber parts is known. To date, suchcompression molding has not be applied for the manufacture of centrifugerotors.

SUMMARY OF THE INVENTION

A method and apparatus for the compression molding of composite fiberfixed angle rotors is disclosed. A female mold member defines a closedcylinder cavity for molding the bottom surface of the rotor, this cavityusually defining a frustum shaped central cavity complimentary to andconcentric with the spin axis of the ultimately formed rotor. A malemold member having a complimentary cylindrical profile contains afrustum shaped inner cavity with the apex of the frustum disposed to theinner portion of the cylinder and the base end of the frustum exposed tothe cylindrical opening of the female mold. This frustum shaped innercavity defines the exterior frustum shape of the ultimately producedrotor and defines between the exterior frustum profile and the frustumshaped inner cavity a rotor body wall having sufficient thickness toreceive the sample tube apertures. At the apex end of the frustum shapedcavity in the male mold member, there is located a locking system formaintaining sample tube aperture cores. These sample tube aperture coresare locked within the frustum cavity in the precise alignment of theultimately formed sample tubes of the rotor. Loading of the mold withresin pre-impregnated fiber typically occurs in the frustum shapedcavity of the male mold member and at the bottom of the female moldmember. Sheet molding compound--flat strips of resin impregnateddiscontinuous fibers--are pre-cut and placed within the mold with theplane of the material normal to the spin axis of the ultimately producedrotor. Reinforcement either with composite cloth, tape, or pre-wound andcured fibers can likewise be loaded with fiber alignment anticipatingthe strength characteristics of the ultimately produced rotor. Withpre-heating, ramped heating to curing temperatures accompanied by rampedcompression of the male and female mold sections one towards another, arotor is rapidly formed in about one hour. Upon rotor formation, thesample tube aperture cores are released from the male mold section, themale and female mold sections parted, and the molded rotor withdrawn.Thereafter, the sample tube aperture core members are individuallywithdrawn, leaving the net shape compression molded rotor.

It will be understood that compression molding imparts the ability tomaintain a high fiber to resin ratio in the ultimately produced rotor.Rotors having high fiber content capable of withstanding the forces ofcentrifugation are produced.

It is further possible to load the mold with pre-cured fiber parts. Inone embodiment, pre-wound fiber rings are added between the frustumshaped mold exterior and the locked sample tube aperture cores to bothreinforce the ultimately produced rotor and to assist in supporting thesample tube aperture cores against the considerable forces encounteredduring compression molding.

In the compression molding of the sheet molded composite discontinuousmaterial, the discontinuous fibers are disposed normal to the spin axisof the rotor before the rotor is molded. As the rotor is molded, thesefibers conform to the molding forces but maintain there generalalignment normal to the spin axis of the rotor. Fibers flow around thesample tube aperture cores radially and from below the sample tubeaperture cores. There results a centrifuge rotor having discontinuousfiber where the fibers are aligned in the finally produced rotor foroptimum resistance to the forces of centrifugation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates exactly one half of the male mold member and thefemale mold member showing the male mold member overlying the femalemold member with the sample tube aperture cores attached within thefrustum shaped cavity of the male mold member;

FIG. 2 illustrates the male and female mold members of FIG. 1 filed withresin impregnated composite material and placed under compression toform a net shaped composite rotor body;

FIG. 3A illustrates the male and female mold members of FIG. 2 inexploded relationship one apart from the other showing both mold membersloaded with composite materials here illustrating the male mold memberassembled and the female mold member having sheet molding compoundplaced within the bottom portion of the open cavity;

FIG. 3B illustrates the sample tube aperture cores attached to oneanother in the molding relationship with the frustum shaped peripherybeing warped with material selected from the group consisting ofunidirectional tape, woven composite fabric, or sheet molded compound;

FIG. 4A illustrates the male and female mold members of FIG. 3A loadedwith composite materials here illustrating the male mold memberassembled and having a fabric winding at the bottom of the sample tubeaperture cores;

FIG. 4B illustrates the sample tube aperture cores attached to oneanother similar to FIG. 3B with the frustum shaped periphery and thelower portion of the sample tube aperture cores being wound withmaterial selected from the group consisting of unidirectional tape,woven composite fabric, or sheet molded compound;

FIG. 5 is a view similar to FIG. 3A illustrating the loading of thefemale mold member with laminates of materials chosen from the groupsconsisting of sheet molded compound, resin impregnated tape and resinimpregnated woven cloth;

FIG. 6A is a view similar to FIG. 3A of the male and female mold membershere illustrating the sample tube aperture cores with tubularly woundbraided composite material for molding the braided composite materialintegrally with the rotor body;

FIG. 6B illustrate a single sample tube aperture core having thetubularly wound braided composite material wrapped about the sample tubeaperture core before installation to the core cluster illustrated inFIG. 6A;

FIG. 7A illustrates the male and female mold members similar to FIG. 3Ahere illustrating the installation of pre-wound and cured fiber rings tothe male mold member, the pre-wound and cured fiber rings being placedbetween the clustered sample tube aperture cores and the frustum shapedcavity in the male mold member;

FIG. 7B illustrates a section of the male mold member taken radially ofthe male mold member of FIG. 7A here illustrating the ring placementbetween the frustum shaped conical cavity of the male mold member andthe sample tube aperture cores, it being noted that the ring member fitsinto preformed steps interior of the frustum shaped cavity of the malemold member to help brace the sample tube aperture cores in their angledclustered relationship;

FIG. 7C illustrates a ring segment for placement between a sample tubeaperture core and the frustum shaped cavity interior of the male moldmember;

FIG. 7D illustrates the ring segment placed between a sample tubeaperture core and the frustum shaped cavity interior of the male moldmember;

FIG. 7E is a detail of a release member for the sample tube aperturecores;

FIG. 8A is a perspective view of a sample tube aperture core clusterhere illustrating the cluster surrounded by pre-wound and cured fiberrings, the fiber rings being keyed to and supported from fiber materialto be pressure molded, this material being placed between the sampletube aperture cores;

FIG. 8B is a radial section taken from the cluster of FIG. 8Aillustrating resin impregnated composite material having at least somevertically oriented fibers with the resin impregnated material beingprovided with notched keyways for keying the pre-wound and cured fiberrings into proper vertical relationship relative to the compressionmolded rotor core to be formed;

FIG. 8C is a radial section taken from the cluster of FIG. 8Aillustrating the section of the rotor body at the sample tube aperturecores illustrating the placement of the pre-wound and cured fiber ringsbetween the sample tube aperture cores and the frustum shaped cavity ofthe male mold member for bracing the core cluster in proper relationshipduring compression molding;

FIG. 9A illustrates the net shaped rotor body when removed from the maleand female mold members of FIG. 2 after compression molding, it beingnoted that the rotor body here illustrated has been provided withperipheral steps to enable convenient fiber winding of the frustumshaped periphery of the rotor;

FIG. 9B illustrates the net shaped rotor body of FIG. 9A wound withcontinuous fibers to provide hoop stress resistance to radial forcesgenerated during centrifugation;

FIG. 10A illustrates sheet molded compound in section so that theimparting of directional configuration during compression molding can beunderstood;

FIG. 10B illustrates pre-impregnated directional composite material tapecut for compression molding as by placement in the female mold member ofFIG. 3A;

FIG. 10C is a view similar to FIG. 10B illustrating pre-impregnatedwoven fabric with composite material fibers cut for compression moldingas by placement in the female mold member of FIG. 3A;

FIG. 10D is an expanded detail of a radial section taken through therotor body of FIG. 9A illustrating the fiber alignment of the curedsheet molding compound illustrating the absence of layering andillustrating the primary normal orientation of the discontinuous fibersto the spin axis of the rotor body with only minor vertical excursion ofthe random fibers;

FIG. 10E is an expanded detail of a section taken through one of thesample tube apertures illustrating the lack of fiber breaking in fiberabutment to the sample tube cores and illustrating typical fiberconformance at the sample tube cores;

FIG. 10F is a detail of the fiber alignment at the exterior of the rotorbody where wrapping of the frustum shaped surface of the rotor bodyoccurs with fabric warp over a central core composed of sheet moldingcompound as illustrated in FIG. 3B;

FIGS. 11A-11D illustrates alternate sample tube aperture cores of thisinvention illustrating the principle that with molded rotor bodies it isno longer required that sample tube apertures be only of cylindricalconfiguration with FIG. 11A illustrating a triangular sample tubeapertures section, FIG. 11B illustrating an elliptical sample tubeaperture section, FIG. 11C illustrating a pie shaped sample tubeaperture section, and FIG. 11D illustrating a pyramid sample tubeaperture section; and,

FIG. 12 is a section of a compression molded rotor having a ringconfiguration suitable for use with that centrifuge shown and disclosedin Centrifuge Construction Having Central Stator Attorney Docket No.16532-5--Ser. No. 08/288,387 filed Aug. 10, 1994 now U.S. Pat. No.5,505,684 issued Apr. 9, 1996.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, male mold member M is shown overlying female moldmember F. Neither mold member is charged with material to be compressionmolded. The configuration of the respective mold members will be setforth first; the operation of the respective mold members will bethereafter discussed.

Taking female mold member F, which includes mold member or forging 12having cylindrical bore 14 for fitting to cylindrical contour 16 of malemold member M. Sufficient clearance is provided between cylindrical bore14 and cylindrical contour 16 so that resin only and not significantamounts of fiber can escape from the joined, compressed, heated andvibrated male mold member M and female mold member F during compressionmolding of a rotor body.

Female mold member F must define the lower contour of the rotor bodyultimately formed. Consequently, it includes male frustum protrusion 18having apex circular surface 20 with base 22 integral with the femalecavity of the mold. Female mold member F is completed with ring surface24, cambered surface 26, and step surface 28. As is conventional,gathering surface 30 is provided at the top of cylindrical bore 14 offemale mold member F.

It will be understood that during compression molding, heating,application of a vacuum, and vibration are utilized. Accordingly,vibrator V, heater H, and vacuum pump U are all schematically shown. Assuch members are conventional, they will not be further illustrated ordiscussed herein.

Having set forth female mold member F, male mold member M will now bediscussed.

Male mold member M includes frustum shaped central cavity C and sampletube aperture core cluster K.

Frustum shaped central cavity C is relatively easy to understand. Itincludes a plurality of machined internal female steps S following thefrustum profile of frustum shaped central cavity C. These internalfemale steps S will be shown later to leave corresponding male steps Tin the ultimately formed rotor body B (See FIG. 9A). Thus, the processof compression molding here disclosed will be understood to result inthe so-called "net shape" or finished state of rotor body B.

Sample tube aperture core cluster K is some what more complicated.Cluster K here includes six sample tube aperture cores R. This numbercan vary to greater or less numbers of sample tube aperture cores R.Referring for example to FIG. 7B, sample tube aperture cores R can beunderstood. Those having experience with centrifugation will understandthat fixed angle rotors are required to have sample tube apertures A(See FIG. 9A). In the compression molding process here described, it isthe function of sample tube aperture cores R to form sample tubeapertures A during the compression molding process. For this to occur,sample tube aperture cores R must be properly held in sample tubeaperture core cluster K before and during compression molding andconveniently removable after compression molding has occurred.

Continuing with FIG. 7B, each sample tube aperture core R includes malecylindrical body portion 40 having relieved bottom surface 42 andcircular bottom 44.

The upper portion of each sample tube aperture core R includes frustumshaped upper end 46 with rounded apex 48. The structures of each sampletube aperture core R is completed with circular segment notch 50. It isthe purpose of circular segment notch 50 to enable the respective sampletube aperture cores R to be held in sample tube aperture core cluster Kduring the compression molding process.

Viewing FIG. 7A, the function of male mold member M at frustum shapedcentral cavity C to hold sample tube aperture cores R in sample tubeaperture core cluster K can be easily understood. At the upper portionof male mold member M in frustum shaped central cavity C, central malemold aperture 52 is configured. Machined at six equal angular intervalsaround central male mold aperture 52, there are frustum shaped coreretaining apertures 36. These respective frustum shaped core retainingapertures 36 each receive and hold frustum shaped upper end 46 of eachsample tube aperture core R.

It remains to securely hold the respective sample tube aperture cores Rin sample tube aperture core cluster K during the compression moldingprocess. Specifically, keying disc 34 fits interior of circular segmentnotch 50 on each sample tube aperture core R. Keying disc 34 is urgedupward by gathering disc 32. Such upward urging occurs through attachedgathering shaft 33 which is typically urged upward by standard threadingor other conventional apparatus (schematically shown). There resultssample tube aperture core cluster K held together with relatively greatforce sufficient to withstand the dynamic forces of compression molding.

It will be understood that once compression molding is finished, releaseof sample tube aperture cores R from the formed sample tube apertures Aand rotor body B is required. To effect such release, male mold member Mand female mold member F are first parted. Once this has occurred,keying disc 34 is rotated. Upon rotation, keying slots 54 in keying disc34 register to circular segment notch 50 in each sample of the tubeaperture cores R. Thereafter, rotor body B is withdrawn from frustumshaped central cavity C of male mold member B. (See FIG. 7E) Keying disc34 can thus be removed. It only remains that the respective sample tubeaperture cores R are removed from the now formed sample tube apertures Ato compete the net shaped rotor body B illustrated in FIG. 9A.

One factor related to the difference between compression molding asillustrated herein and injection molding should be emphasized. We havefound that it is required that centrifuge rotors have high fiber contentto withstand the considerable forces of centrifugation. This being thecase, a high fiber content--in the order of 50% of the weight percent ofthe resin fiber mixture is required. Such a high fiber content materialis absolutely unsuitable for injection molding; injectors cannotconveniently handle or inject a resin/fiber mixture with such a highfiber content.

Further, we do not here rely on so-called resin transfer molding. Thatis to say, we do not charge the mold first with totally unimpregnatedfiber and thereafter inject resin without supplying the considerablecompression forces here illustrated. Such molding would have thepossibility of leaving voids in rotor body product which wouldultimately render the final product not suitable for centrifugation.

It will be understood that we show female mold member F underneath malemold member M. This can be reversed. Further, a vertical relativedisposition between the respective portions of the mold is not required.For example, the mold members could move horizontally towards and awayfrom one another--although this is not preferred.

Having set forth the mechanics of the mold, the loading of thecompression mold with material for compression molding can now bediscussed in detail.

Referring to FIGS. 3A and 3B, a first loading of male mold member M andfemale mold member F can be understood. In this embodiment, resinimpregnated composite fiber precut discs 60 cover the bottom ofcylindrical bore 14 in female mold member F. As can be seen, theserespective resin impregnated composite fiber precut discs 60 extend overmale frustum protrusion 18 to step surface 28 at the bottom ofcylindrical bore 14 of female mold member F. It is preferred that theserespective resin impregnated composite fiber precut discs 60 consist ofpreferably of sheet molding compound. They can be chosen from the groupincluding sheet molded compound, pre-impregnated composite fiber tape,or pre-impregnated composite fiber fabric.

Overlying resin impregnated composite fiber precut discs 60 there areplaced central fiber layers 62. Central fiber layers 62 are preferablyformed from sheet molded compound 65. Some discussion of thiscommercially available compound and its applicability is warranted.Accordingly, the reader's attention is directed to FIG. 10A.

Referring to FIG. 10A, it can be seen that such sheet molded compound 65consists of chopped fiber layers 67 alternating with resin layers 68.This alternating construction can be found in SMC produced by QuantumComposite of Midland, Mich.

Referring briefly to FIG. 9A, it will be understood that when rotor bodyB spins about rotor spin axis 70 at high rotational velocity, majorstress will be exerted normal to the spin axis. It will be understoodthat if the discontinuous fibers 69 illustrated in FIG. 10A could beoriented overall substantially normal to rotor spin axis 70, rotor bodyB would have maximum resistance to the forces of centrifugation.

Referring back to FIG. 3A, and central fiber layers 62, it will beunderstood that these respective layers are preferably made of sheetmolded compound 65. It has been found that during molding, therespective discontinuous fibers 69 of central fiber layers 62 maintainthere respective major horizontal disposition normal to rotor spin axis70 of the ultimately formed rotor body B. Further, upon curing in thecompression molding process here disclosed, the respective layering seenin FIG. 10A, is no longer visible. Instead, the respective discontinuousfibers 69 have major alignment normal to rotor spin axis 70 but form inthe net shape rotor body B without any apparent layering being present.

It should be further understood that when sheet molded compound 65 ismolded, some vertical orientation of discontinuous fibers 69 occurs.This vertical orientation imparts to rotor body B resistance to verticalforces placed on the rotor during centrifugation. For example, sampletubes within sample tube apertures A can exert a considerable force onthe respective bottoms of the sample tube apertures A. Where the rotoris made of composite fiber layers normal to rotor spin axis 70, suchcomposite fiber layers have been known to delaminate under suchcentrifugation generated forces. It has been found that sheet moldedcompound 65 and the minor vertical orientation of discontinuous fibers69 advantageously resists such forces.

Finally, it should be understood that during compression molding,central fiber layers 62 when made of sheet molded compound 65 have theadvantage of readily deforming and conforming intimately about the shapeof female mold member F and particularly the more intricate threedimensional configuration of frustum shaped central cavity C withcentral sample tube aperture core cluster K. It is for this reason thatin the embodiment illustrated in FIGS. 3A and 3B, it is preferred tohave central fiber layers 62 made from sheet molded compound 65.

It will be understood that dependent upon the overall strength of thefinally manufactured rotor body B, other materials may be addedinteriorly of the mold. For example micro-balloons (glass, phosphor, orcarbon) can be added. Additionally, and dependent upon the stresslocation in rotor body B, materials such as ordinary fiber glass may beused.

Referring to FIG. 3B, wrapping of sample tube aperture core cluster Kwith resin impregnated fiber layer 64 is illustrated. Such wrapping hereconsists of resin impregnated woven fabric. It will be understood thatother materials could be used including woven composite fabric notimpregnated with resin, composite tape (optionally resin impregnated),or sheet molded compound.

Once the particular female mold member F and male mold member M arerespectively loaded, compression molding can occur. With mold separationas previously described, rotor body B as illustrated in FIG. 9A results.

The remaining portions of the description herein will assume thatcompression molding occurs. Those have skill with composite fibers andresins will realize that the temperatures, pressures and durationrequired in curing will vary with the resin system mixture involved.While this requires considerable testing when new formulations areutilized with particular molds, persons having skill in the curing ofcomposite fibers impregnated with resins can readily determine suchparameters.

Referring to FIGS. 4A and 4B, sample tube aperture core cluster K iswrapped at each sample tube aperture core R with portions of dependingcomposite material wrap 72. Depending composite material wrap 72 is slitat intervals between sample tube aperture cores R and has the slitportion extending below sample tube aperture core cluster K wrappedabout the lower portion of each sample tube aperture core R. It will beappreciated that this configuration when molded about sample tubeaperture cores R produces sample tube apertures A having composite fiberreinforcing the bottom of the apertures A. It will be understood thatsuch sample tube apertures A have high resistance to the force of sampletubes bearing vertically downward at the bottom of the respective sampletube apertures.

FIG. 5 illustrates a loading of female mold member F with sheet moldingcompound rings 75 and sheet molding compound discs 77. Unlike theexample previously given in FIG. 3A, reliance is placed upon sheetmolding compound discs 77 to conform around sample tube aperture cores Rwhen in the fluid state under compression molding. This phenomena can bereadily understood.

Specifically, as sheet molding compound rings 75 and sheet moldingcompound discs 77 are heated, compressed and vibrated, the laminatestructure of the cut material is lost. The respective fibers withinrings 75 and discs 77 conforms around sample tube aperture cores R asheld in sample tube aperture core cluster K. Unfortunately, this willinterfere with some of the normal alignment of the fibers with respectto rotor spin axis 70 (See FIG. 9A). It does have the advantage ofcausing many fibers to conform to the surface of sample tube aperturecores R and thus form sample tube apertures A having fibers disposed inthe plane of the surface of the sample tube apertures.

Individual reinforcement of sample tube apertures A is possible alone orin combination with the other techniques mentioned herein. Referring toFIGS. 6A and 6B, the respective sample tube aperture cores R are shownwrapped in composite fiber cloth sock 80. Composite fiber cloth sock 80can be either pre-impregnated or alternate "dry", in which case relianceon acquiring resin from adjacent pre-impregnated fiber is required. Itwill additionally be appreciated that respective composite fiber clothsocks 80 can be either fully or partially cured before placement ontheir respective sample tube aperture cores R.

Referring to FIGS. 7A and 7B, integral molding of a rotor body isillustrated where wound and pre-cured fiber tension rings 85 areutilized. Specifically, and referring to FIG. 7A, wound and pre-curedfiber tension rings 85 are wound with a diameter to fit into internalfemale steps S. These respective rings 85 fit to the internal portion offinished rotor body B and leave exteriorly thereof the respective malesteps T seen in FIG. 9A. Thus it is possible to internally reinforcerotor body B.

Referring to FIG. 7B, an additional advantage of wound and pre-curedfiber tension rings 85 can be seen. Examining male mold member M atfrustum shaped central cavity C in the interface between internal femalesteps S and sample tube aperture cores R, it will be seen that wound andpre-cured fiber tension rings 85 occupy the interface precisely. That isto say, wound and pre-cured fiber tension rings 85 contact internalfemale steps S on one side and sample tube aperture cores R on the otherside. With this configuration, the proper angularity of sample tubeaperture cores R in sample tube aperture core cluster K is assured, evenin the presence of the considerable forces encountered duringcompression curing of the resin and fiber. Thus wound and pre-curedfiber tension rings 85 have a secondary function in bracing sample tubeaperture cores R relative to male mold member M at frustum shapedcentral cavity C.

It is also possible to use ring segment 90 for this same effect.Referring to FIGS. 7C and 7D, ring segment 90 is shown backing sampletube aperture core R relative to internal female steps S of male moldmember M.

Referring to FIGS. 8A-8C, it is possible to reinforce the compressionmolded rotor body with wound and pre-cured resin fiber rings 105. Thiscan be done when frustum shaped central cavity C is configured withoutinternal female steps S or with smooth frustum shaped internal femalesurface 106. Referring specifically to FIG. 8C, it will be seen thatwound and pre-cured resin fiber rings 105 occupy the interstitial volume107 between sample tube aperture cores R and frustum shaped centralcavity C interior of male mold member M. At the same time, and referringto FIGS. 8A and 8B, it will be seen that pre-impregnated fiber mass 100fits between respective sample tube aperture cores R. In this location,it is possible to use pre-impregnated fiber mass 100 with ringsupporting notches 102 to support respective wound and pre-cured resinfiber rings 105. Thus, with frustum shaped central cavity C of male moldmember M charged with sample tube aperture core cluster K,pre-impregnated fiber mass 100 and wound and pre-cured resin fiber rings105, a rotor body without male steps T can be fabricated which has ringreinforcement. Such a rotor body requires no further finishing.

Referring to FIG. 9B, finishing of rotor body B is illustrated.Specifically, resin fiber windings W are tension wound and cured overmale steps T. Such winding and curing secures under tension resin fiberwindings W to the exterior surface of rotor body B to provide additionalresistance against the forces of centrifugation.

It will be apparent that the illustrated molding process providessuperior flexibility. Specifically, and using the compression moldingcavities here illustrated, all shapes of fibers can be compressionmolded. For example, and referring to respective FIGS. 10B and 10Csuccessive angularly alternated impregnated tape 110 or successiveangularly alternated woven fiber 120 can be pre-impregnated andcompression molded in female mold member F here illustrated. Likewise,this invention will admit of other variations to the compression moldingherein set forth.

Compression molding is known.

Observation of the compression molded part is helpful. It is common inmetallic centrifuge rotors to forge metal blanks or "forgings" for suchrotors. When such forging occurs, and the metal resulting from suchforging is microscopically examined, especially as to the granularstructure, the metallic grains can be treated so as to be optimallyaligned to resist the forces of centrifugation.

It is to be understood that the present process of compression moldingis analogous when it comes to alignment of fibers with respect to theresultant compression molded rotor body B as seen in FIGS. 9A and 9B.Specifically, and when sheet molded compound 65 is utilized, fibersnormal to the spin axis result. (See FIG. 10D) Cutting such a rotor bodyin section results in a view of cured fibers E "flowing" along planesnormal to the spin axis or parallel to horizontal vector 130. Such aplane normal to rotor spin axis 70 is illustrated by spin axis normalvector 131. It will be observed that no measurable kinking is present insuch fibers. Moreover, only in minor detail do the fibers depart fromthe original alignment of sheet molded compound 65. Taking the case ofvertical vector 131 disposed parallel to rotor spin axis 70, it will beobserved that individual fiber excursion from the plane of spin axisnormal vector 130 is small, yet present in sufficient amount to enablesufficient vertical strength to be present in the rotor body to resistthose forces generated parallel to rotor spin axis 70. This appearanceis distinctive and is illustrated herein at FIG. 10D.

Referring to FIG. 10E, when fibers E encounter a mold boundary, againthe appearance is distinctive. First, there is little evidence of thefibers being sheared. Second, the fibers present a checkered almost"marbleized" appearance when seen with the eye. Finally, the fibersalign themselves parallel to the surface which they encounter at theboundary of a mold.

Referring to FIG. 10F, it is preferred to place fabric around thefrustum shaped periphery of the mold as illustrated in FIG. 3B. Takingthe case where resin impregnated fiber layer 64 is made from resinimpregnated fabric, the fabric clearly appears only at the boundary.Such resin cured woven composite fabric 164 is illustrated in FIG. 10F.It will be noted that the interior of sample tube apertures A whenformed with composite fiber cloth sock 80 around sample tube aperturecores R has a similar appearance.

It is to be emphasized that for the first time, a compression moldedrotor body B is produced. No longer is it required that sample tubeapertures A be machined. Specifically, they can now be molded. And moreimportantly, they can be molded to shapes that are other thancylindrical.

Referring to FIG. 11A, a cross section of sample tube aperture A havinga triangular cross section 140 relative to rotor spin axis 70 isillustrated.

Referring to FIG. 11B, a cross section of sample tube aperture A havingan elliptical cross section 142 relative to rotor spin axis 70 isillustrated. It will be noted that the major axis of the ellipse isradially aligned relative to rotor spin axis 70.

Referring to FIG. 11C, a cross section of sample tube aperture A havinga pie shaped cross section 144 relative to rotor spin axis 70 isillustrated. It will be noted that the apex of the pie shape is disposedtowards rotor spin axis 70.

Finally, and referring to FIG. 11D, a cross section of sample tubeaperture core R having pyramid shaped three dimensional section 146 isillustrated. It goes with out saying that such a sample tube aperturecore R will form sample tube aperture A having a complimentary femalecross section.

Observing FIGS. 11A-11D, some observations can be made about the variedsample tube apertures A. First, they are molded and all other thancylindrical. Second, it is required that sample tube aperture cores Rall be capable of releasing from the mold. Such release here is shown inits preferred embodiment from male mold member M. It will be understoodthat release from female mold member F could likewise occur. Further,mold members with straight sample tubes--parallel to the direction ofmold release--will not require release of sample tube aperture cores R.

In co-pending Centrifuge Construction Having Central Stator, Ser. No.08/288,387 filed Aug. 10, 1994, now U.S. Pat. No. 5,505,684 issued Apr.9, 1996, inventor Piramoon has disclosed the construction of a newcentrifuge. Specifically, this centrifuge contains a central statorwhich produces a rotating magnetic field. The peripheral rotor couplesto this rotating magnetic field. It will be understood that toaccommodate the central stator, some section of the ultimately producedrotor body B₁ has to be ring shaped. Such a ring shaped rotor body B₁ isillustrated in FIG. 12.

Referring to FIG. 12, rotor body B₁ includes a central stator aperture150, and maximum capacity shaped sample tube apertures 152. Some commentis in order.

First, the molding apparatus here illustrated can be modified to makeany shape of rotor body B. We prefer the fixed angle embodiment of rotorbody B as of this time. It will be understood that with the introductionof additional centrifuges, other rotor bodies may be required such asrotor body B₁ having central stator aperture 150.

Secondly, we now understand that rotor body B₁ first disclosed inCentrifuge Construction Having Central Stator Ser. No. 08/288,387 filedAug. 10, 1994 now U.S. Pat. No. 5,505,684 issued Apr. 9, 1996, hasseveral advantages over conventional spindle mounted rotors. First, itrequires a larger diameter. This results in a lower speed of rotation.Further, a greater number of sample tube apertures A can beaccommodated. For example the reader will observe eight sample tubeapertures A in FIG. 12.

Secondly, it is especially advantageous to change of the shape of sampletube apertures A to maximize capacity of the sample tube apertures andany tubes which are subsequently placed within them. As such rotor bodyB₁ is conventionally reinforced by resin fiber windings W, the maximumcapacity shaped sample tube apertures 152 do not appreciable detractfrom the overall rotor resistance to the forces of centrifugation.

It will therefore be understood that the enclosed described compressionmolded rotor has wide applicability.

What is claimed is:
 1. An apparatus for compression molding a fixedangle centrifuge rotor body having a frustum shaped peripheral contourabout a central spin axis between a base end and an apex end, the rotorbody having angled sample tube apertures extending from openings in theapex end adjacent the spin axis of the rotor body to bottom portions ofthe sample tube apertures more remote from the spin axis of the rotorbody, the mold apparatus comprising:a mold member; a frustum shapedcavity interior of the mold member with a periphery having the frustumshaped peripheral contour of the rotor body; sample tube aperture cores,each sample tube aperture core for forming a sample tube aperture; meansfor clustering the sample tube aperture cores to form a cluster ofsample tube aperture cores to form openings for sample tube apertures inthe apex end of the rotor body adjacent the spin axis to bottom portionsof the sample tube apertures more remote from the spin axis of the rotorbody; and, means for mounting the cluster interior of the frustum shapedcavity of the mold member.
 2. An apparatus for compression molding afixed angle centrifuge rotor body according to claim 1 and wherein themeans for mounting the cluster interior of the frustum shaped cavity ofthe mold member includes:means for release of said sample tube aperturecores from the cluster.
 3. An apparatus for compression molding a fixedangle centrifuge rotor body according to claim 1 and wherein said moldapparatus further includes:the mold member having a male contour with acavity defined therein; a second mold member having a female contour forreceiving the male mold member with the cavity exposed to the femalemold.